Cyanoacrylate-based adhesives have been found useful in a wide array of industrial and domestic applications involving bonding of two or more objects. When used in conjunction with catalysts such as aromatic amines that accelerate the anionic polymerization process and resultant curing of the adhesive, the cyanoacrylate-based adhesives are noted for high bonding strength, rapidity of curing and durability. These qualities have made cyanoacrylate-based adhesives the adhesives of choice in such diverse applications as assembly of electrical and computer components, assembly of model airplanes, ships and like objects by hobbyists, and attachment of silk screens to frames in the silk screen printing industry.
The adhesive-based attachment of silk screens to frames is illustrative of the advantages and disadvantages of prior art adhesive compositions and methods of application. In the silk screen serigraphic printing process, various colored inks or dyes are transferred through a pattern of openings or holes in a screen fabric onto an object to be printed. The pattern of openings may be fashioned by selectively blocking or occluding a desired array of openings in the screen fabric, leaving a pattern of unoccluded openings representing the selected print design. The movement of ink or dye through the openings onto the printed substrate creates a printed design on the substrate that duplicates the pattern of unoccluded openings in the screen fabric.
In order to provide an accurate and reproducible transfer of ink through the screen fabric onto the printed substrate, it is necessary for the screen fabric material to be in a taut configuration. As such, the screen material is generally stretched by securing the periphery of the material to a frame structure. The frame can be of any desired shape, but is frequently manufactured so as to maintain the screen fabric in a planar configuration. In a common configuration, the frame structure provides a square or rectangular configuration similar to that of a typical picture frame, although with additional rigid supporting structure beneath the screen-engaging portion of the frame. The screen fabric is bonded or otherwise attached to the periphery of the screen-engaging portion of the frame such that uniform tension is applied to the fabric in the planar dimension. That is, tension is applied in directions corresponding to the four sides of the frame as described above.
In addition to the commonly used screen printing fabrics, a variety of other fabrics may be employed for screen printing, including without limitation polyesters, nylons, stainless steel, cotton and similar screened materials. Depending on the desired application the fabrics may possess various mesh sizes and densities, i.e., may possess various sizes of openings as well as various densities of openings per unit area.
Typically, the screen fabric in taut condition is contacted with the screen-engaging peripheral structure of the frame. Next, the fabric is bonded to the frame, with the goal being duplication and maintenance of the original taut condition. With older wooden frames, it was possible to staple the fabric to the frame. With modern frames manufactured of metal or other non-wooden materials, staple attachment is not an option. Consequently, adhesive bonds became widely used as non-wooden frames entered the industry. For purposes of bonding, a common practice has been to apply, from a squeeze-type bottle for example, a selected quantity of cyanoacrylate-based adhesive to that portion of the screen in contact with the peripheral screen-engaging portion of the frame. The adhesive is spread out with a card or other suitable spreader to ensure proper contact of the adhesive with all portions of the fabric in contact with the frame. Although the cyanoacrylate-based adhesive will slowly cure upon contact with the moisture content of air, the curing process (anionic polymerization) generally is accelerated with a catalyst. The catalyst is generally a basic chemical compound, and aromatic amines have proven particularly useful as efficient catalysts.
Various alkyl-substituted cyanoacrylate adhesives have been used in the screen printing industry. The nature of the alkyl group influences the viscosity, bonding and other characteristics of the adhesive. For example, methylcyanoacrylates generally exhibit a low viscosity suited for screen fabrics having a relatively small mesh size. Ethylcyanoacrylates generally display a higher viscosity than the methylcyanoacrylates and are therefore more suited to screen fabrics of somewhat larger mesh size. Since it is often desirable to re-use the frame for a succession of printing jobs, it is desirable to form an adhesive bond that allows the screen fabric to be peeled away from the frame while at the same time providing relatively high sheer strength in order that the fabric remains appropriately taut in use. Isopropylcyanoacrylates, which display even greater viscosities than the ethylcyanoacrylates, and are appropriate for applications requiring a bond of particularly high peel strength. This may be necessary, for example, when printing occurs in particularly close proximity to the edge of the screen fabric.
Following application of the cyanoacrylate adhesive, the catalyst is applied, generally as a solute in a solvent carrier. In one commonly used method of application, as disclosed for example in U.S. Pat. No. 4,702,783, an aromatic amine catalyst is applied as a spray or freon-propelled aerosol directed at those portions of the screen and underlying frame to which the cyanoacrylate adhesive has been applied. The goal of such application is provision of a light coat of the spray catalyst so as to provide a satisfactorily polymerized bond in 30 seconds or less. The amount of catalyst applied may need to be varied depending on the amount of cyanoacrylate adhesive previously applied to the substrate and the temperature of the catalyst solution and substrate.
A problem with cyanoacrylate adhesives is that such adhesives typically are clear in appearance. As such, it is frequently difficult to ascertain both the precise amount of adhesive applied as well as the precise boundaries of the adhesive-coated regions of the substrate. Similarly, the catalyst solution also may be clear in appearance, such that it may be difficult to monitor the amount of catalyst solution applied during the application process and to ensure confinement of the catalyst solution to desired areas of substrate. This is particularly true for propelled aerosol application of catalyst solution.
If an excess amount of adhesive is applied to the substrate, if the spreading of the adhesive is uneven, or if an excess amount of catalyst solution is applied, all or parts of the bond may be defective due to entrapment of solvent in the cured adhesive. Typically such a bond has a whitish filmy appearance, and such "blushing effect" or "blooming effect" is frequently perceived as evidence of a failed bond. Conversely, application of insufficient adhesive or catalyst also may result in defective bonds. One approach to the problem of a clear-appearing adhesive compound has involved addition of various dyes to the adhesive, as exemplified in U.S. Pat. No. 4,702,783. This allows for visual estimation of the amount and location of cyanoacrylate adhesive. In addition, catalysts are often dissolved in highly volatile solvents (for example, solvents with boiling points of approximately 100.degree. C. or less) with the intent that the solvent is able to evaporate before becoming entrapped in the polymerizing adhesive. On the other hand, highly volatile solvents may chemically diffuse into the cyanoacrylate adhesive with adverse effects on the bonding characteristics
Unfortunately, the catalyst solutions as employed in the art have had less than optimum properties in regard to environmental and health concerns. Volatile organic solvents (for example, solvents which at room temperature exhibit vapor pressures greater than approximately 50 mm Hg) may have a variety of toxic effects when inhaled or when brought into skin contact. This is especially so when catalyst solutions are applied as aerosols, which may expose industry workers not only to volatilized solvents but also to volatilized catalyst compounds which themselves may have adverse health consequences. Previous industrial methods of application of catalyst solution may also have adverse environmental effects. Industrial or domestic use of volatile organic solvents of course leads to release of such solvents into the atmosphere. In addition, use of trichloroethylenes or of fluorocarbons (e.g., freon) as aerosol propellants may contribute to destruction of atmospheric ozone.
Accordingly, it would be useful to provide means for applying effective and controlled amounts of catalyst solution in which the catalyst is dissolved in a biodegradable, low-volatility solvent. To the extent that such means allow for precise delivery of controlled amounts of catalyst solution to defined, or delimited, areas of substrate, such means would prove useful even with volatile solvents.